This invention relates generally to magnetic layered structures for use in magnetic recording media, particularly magnetic recording disks.
Conventional magnetic recording disks have a magnetic recording layer, typically of a ferromagnetic alloy, such as a cobalt (Co) alloy, that is sputter deposited as a continuous thin film having grains of the crystalline magnetic material. It is well known that to achieve high density recording in magnetic recording disks it is necessary to decrease the grain size, increase the coercivity (Hc), and reduce the remanent magnetization-thickness product (Mrt) of the magnetic recording layer. Therefore, most attempts to increase the magnetic recording density of magnetic media have focused on these three parameters by altering the composition and microstructure of the magnetic material in the magnetic layer or by reducing the magnetic layer thickness to achieve low Mrt. Unfortunately, very thin magnetic recording layers with small grains can become thermally unstable, wherein the magnetic moments of the small grains can spontaneously switch their magnetization direction, resulting in loss of the recorded data. This is of concern, especially at the elevated operating temperatures of the disk drive. Thermally unstable grains, at sufficiently small grain sizes, are generally referred to as superparamagnetic grains.
These superparamagnetic grains represent a serious obstacle to further increases in magnetic recording densities. This limitation of conventional media is evident by the fact that a reduction in grain volume by a mere factor of one half, for instance, may change the thermal stability from being on the order of several years to less than a minute.
Conventional media may show amplitude loss, noise increase, increased data error rate, loss of resolution, etc., in short, a general loss of magnetic recording performance due to thermally driven demagnetization processes even before catastrophic and rapid thermal demagnetization sets in at the superparamagnetic limit. This slower but steady magnetization decay can be characterized by a number of experimental measurement methods, including magnetic xe2x80x9cviscosityxe2x80x9d (magnetization decay rate) measurements. The conventional magnetic recording media can exhibit significant magnetization decay rates which impose serious limitations on the magnetic recording densities that can be achieved.
The conventional mechanism to stabilize thermally activated magnetization processes is by raising the magnetocrystalline anisotropy and coercivity of the media. This route is however subject to the constraint that the magnetic write head can produce only a maximum field magnitude which is limited by the magnetic moment density of the material of the pole pieces of the write head. The writeability of high-performance media is critical in order to increase the magnetic recording densities.
J. Chen et al. in IEEE Trans. Mag., Vol. 34, no. 4, pp. 1624-1626, 1998; describes using xe2x80x9ckeeper layersxe2x80x9d for thermally stabilizing the magnetic recording layers in the media. A chromium (Cr) break layer is laminated between the keeper layer and the magnetic layer. For example, a Cr break layer approximately 25 Angstroms thick is deposited on a magnetic layer prior to depositing a relatively thick (50 to 150 nm) keeper layer. The Cr break layer is used to control the growth of the keeper layer and to prevent exchange coupling between grains within the magnetic layer. Depositing such a thick magnetically soft keeper layer directly on a magnetic recording layer will cause strong magnetic inter-granular coupling in the magnetic layer and hinder its use for high density recording. The function of the keeper-layer/break layer construction is to reduce the demagnetization fields from adjacent magnetic transitions. Keeper layers are known to be a large source of recording noise. Furthermore, the keeper layer construction reported are 50-150 nm thick. It is preferred to have the total thickness of the layers and any overlays and protective overcoats making up the medium to be very thin to optimize the recording performance.
What is needed is a magnetic recording medium with a thin magnetic layer that has high coercivity, large coercivity squareness (S*), low remanent product thickness (Mrt), and that is thermally stable and writeable to be able to supporting very high recording densities. It is preferable that such a medium can be fabricated using conventional film deposition methods.
Accordingly, it is a primary object of the present invention to provide magnetic recording media exhibiting enhanced thermal stability. Because the magnetic media of this invention exhibits improved thermal stability, thinner magnetic recording layers containing smaller grains can be used, resulting in higher magnetic data recording densities.
It is a second object of the invention to improve the recording characteristics of very thin magnetic layers by providing ferromagnetic overlays or capping layers deposited on these magnetic layers. The ferromagnetic overlays increase the effective volume of the small thermally unstable grains in the magnetic layers and thus increase thermal stability.
It is a third object of the present invention to provide magnetic recording media with improved writeability.
Lastly, it is an object of the present invention to provide magnetic media with sharp magnetic transitions, high S* and low Mrt.
The objects and advantages of the present invention are obtained by providing a magnetic media with a magnetic recording layer that includes a ferromagnetic xe2x80x9chostxe2x80x9d layer and a thin overlay or capping layer of ferromagnetic material deposited on the host layer. The ferromagnetic host layer is a granular layer with grains that are weakly coupled or uncoupled, wherein the grains are capable of independently changing magnetization directions in the presence of local magnetic fields generated by the magnetic write head. The ferromagnetic overlay enhances the thermal stability and writeability of the ferromagnetic material in the host layer, is substantially thinner than the host layer, and is exchange coupled to the host layer. Ferromagnetic xe2x80x9coverlayxe2x80x9d is meant to refer to a ferromagnetic material that is either a continuous layer of ferromagnetic material, a discontinuous layer in the form of a dispersion of islands or grains of ferromagnetic material, or a multiple phase layer containing islands or grains of a first ferromagnetic material separated by a second non-ferromagnetic material. Because the ferromagnetic overlay can be a discontinuous film or dispersion of material, it is convenient to refer to an xe2x80x9ceffective thicknessxe2x80x9d which is the thickness that would be attained with the same particle flux from the deposition apparatus for a film exhibiting continuous coverage. In addition, because multiple phase materials are also considered as ferromagnetic overlays, it is convenient to refer to effective concentrations of materials, which describe an average concentration of a material in the overlay. When referring to either a ferromagnetic overlay or a ferromagnetic host layer as a continuous layer the intent is to distinguish a continuous layer from a discontinuous layer or a dispersion of material, and is not intended to imply that the layer is a single phase material.
A magnetic medium of the present invention has at least one magnetic Co-based layer that is preferably 1 to 30 nm thick and that is deposited on a prepared substrate. The substrate is any suitable disk substrate, such as an aluminum disk blank coated with nickel phosphorus, glass coated with NiAl, silicon, ceramic, quartz, MgO and silicon-carbide. The substrates are covered with an underlayer in order to achieve the desired crystalline orientation of the subsequently deposited ferromagnetic host layer. Pure Cr is a typical underlayer, but underlayers may also include Cr alloys containing an element of Co, V, Ti and O. The choice of substrates and underlayers is dependent on the ferromagnetic host layer to be deposited and the intended application of the medium.
A ferromagnetic host layer is preferably a continuous magnetic recording layer, but may also be a patterned ferromagnetic host layer that is patterned by any of several methods known in the art to form discrete ferromagnetic and nonferromagnetic regions. In a preferred embodiment, a ferromagnetic host layer is a CoCr ternary or quarternary alloy with Co in the range of approximately 20-85 atomic percent (at %) and Cr up to approximately 30 at %. In a preferred embodiment the ferromagnetic host layer also contains Pt, Ta or Pd in the range of 1-20 at % and a segregating element of boron (B) in an effective concentration of 3-25 at %. Other segregating materials include oxides of Si and Co, and several transition metals, including Ti, Zr, Hf, Ag, Nb, W and Au, that weakly couple or do not couple magnetic grains. Materials used to form the ferromagnetic host layers have saturation magnetization (Ms) values between approximately 50 and 1000 emu/cm3.
The thin ferromagnetic overlay or capping layer containing at least one element selected from Co, Fe and Ni is deposited on the ferromagnetic host layer. The capping layer is preferably a high moment ferromagnetic material with a saturation magnetization (Ms) value between approximately 50 and 1900 emu/cm3. The ferromagnetic overlay is preferably a material that has a significantly higher magnetic moment (e.g., 1.5 times greater) than magnetic moment of the ferromagnetic host layer material. The ferromagnetic overlay is preferably a dispersion of ferromagnetic islands on the surface of the ferromagnetic host layer with an effective thickness of approximately 1-40 Angstroms. In an alternative embodiment, the ferromagnetic islands, also referred to as grains, contain elements of Pt and Pd in the range of approximately 1-75 at % or Cr in the range of 1-35 at %. In particular embodiments of the present invention the ferromagnetic overlay grains are separated by an amorphous secondary phase including CoO, SiO2, or binary or ternary compounds of B.
The underlayers, ferromagnetic host layers and ferromagnetic overlays of the present invention are deposited by any number of methods including sputtering, ion-beam deposition and laser deposition. It is also desirable to deposit a protective overcoat, such as a conventional amorphous carbon overcoat, after depositing the ferromagnetic overlay to prevent oxidation and degradation of the overlay and ferromagnetic host layers. Magnetic media of the present invention have coercivity values ranging between 2-20 kOe.
In general, magnetic volume elements, which comprise multiple grains of the ferromagnetic alloy in the magnetic layer that together form a magnetic bit, contribute more to the readback signal the closer they are to the magnetic read head. In the present invention the magnetic material with the highest moment density, namely the material in the overlay, is placed in closest proximity to the read head. The moment density of Co, for instance, is nearly five times the moment density of the conventional ternary or quarternary Co alloy, which has a moment density of about 300 emu/cm3. Thus a very thin layer of 0.5 nm of Co has nearly five times the moment density of a conventional ternary or quarternary Co alloy. Therefore a very thin layer of 0.5 nm of Co not only has an Mrt equal to that of a layer of 2.5 nm of conventional Co alloy, it also places the source of the magnetic flux in much closer proximity to the read head. And because the high moment density overlay material is placed in closest proximity to the write head there is also an improvement in writeability since during writing the field from the write head is strongest at the surface closest to the head. Furthermore, the magnetic torques that are exerted by the applied field from the write head are amplified by the increased moment density of the ferromagnetic overlay. The present invention therefore offers improvements in writing by combining the strongest write field magnitudes with the largest magnetic torques at the surface of the magnetic recording layer in the medium.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.